It is covered in motorcycle training. On motorways, the legality of it has always been debatable but the police ?allow? it so long as is done with care.
In traffic, the things to watch for are unannounced, all of a sudden right turns or u-turns from cars. Traffic pulling out from the left, (ie they are turning right but will be looking for traffic to their left and not expecting a two wheeled vehicle coming up on the outside). Oncoming traffic (obviously). Doors, it is rare that a driver will throw open their door in traffic but it does happen. Fag ends are a particular problem when they get thrown out of the window but when they are, and they just miss you if you shout ?thanks? straight away as you pass the car, it freaks the hell out of the driver. PS chucking fag ends out of cars is illegal.
Pedal cyclists filtering on the left have additional problems such as oncoming traffic turning right and the traffic they are passing turning left. Getting doored by a passenger getting out is a very real problem. Don?t be fooled by the provision of a cycle lane, a cycle lane in this instance is a very dangerous place. Then if you do get to the front, you need to get in front of the car in front to make sure they see you. If it is a larger vehicle, especially with a high cab, DON?T DO IT, wait behind. For others it was the last thing they did. Don?t be fooled by cycle lanes either, they offer no protection.
And?
Never ever ever (no never) filter on the left if there are railings, even if there is a cycle lane. If the traffic moves left or turns left you will die by crushing.